1. Field of the Invention
The present invention relates to a noise barrier to control or attenuate undesired sounds or noises coming from streets and highways, railways, factories, etc., and more particularly to a noise control module and a method of installing the noise control module.
2. Description of the Prior Art
Conventionally, to attenuate noises from streets and highways, railways, factories or the like, various wall types of noise barrier are prevalently used to block noises from coming directly from such noise sources. This is because the wall-type noise barrier structures are of relatively low cost while working as effectively as various other noise control modalities.
Among the noise control structures, upright or vertical straight wall types of noise barriers are used most widely because they show a high effect of noise control for their relatively low costs and are widely applicable for controlling noises from various sources. For a higher effect of noise control, the vertical straight wall type noise barriers have to be taller. However, such taller ones are correspondingly more expensive (the taller the wall, the greater construction costs they need). In addition, the taller walls have many other disadvantages such as interception of sunlight, obstruction of view (shut-off of prospect), oppressive sensation or claustrophobia, ill ventilation, radio interference or jamming, turbulent flow, wind load, etc.
Using such a vertical straight wall type noise barrier only will not be a satisfactory solution to the noise-control problem in some cases. For an improved noise control, it has been proposed to use various other types of noise barriers including bent-top types of which the wall top portion is bent toward the noise source and curved-top types of which the wall top portion is curved toward the noise source. However, it has also been known that use of a noise barrier of such a bent- or curved-top type will make the above-mentioned problems more serious.
The recent drastic increase of the traffic and higher speed capability of vehicles have resulted in serious environmental noise problem. Since no other effective noise control solutions have been available, the noise barriers of the vertical straight wall type of 3 m, 7 m and 10 m in height and also the above-mentioned bent-top and curved-top types of noise barriers, are used. However, the above-mentioned environmental problems still remain unsolved.
The above conventional noise barriers can only provide an effect of noise attenuation improved for an increased wall height. Generally, at a place (about 20 meters) separate from the noise barrier, an increase by 1 m in height of the wall only results in a noise attenuation of about 1 dB.
To attain a higher effect of noise control without increasing the wall height, a noise barrier structure has been proposed which has a generally Y-type cross section, Y-type being formed by a main soundproof wall and two additional top screens provided on top of the main body and define an upward opening between them. The noise control or attenuation by this Y-type noise barrier was evaluated by measuring noise level at a place behind the barrier when viewed from a noise source. Also a noise barrier of a vertical straight wall type as tall as the Y-type one was evaluated concerning its effect of noise attenuation. Thus, the noise attenuation (measured sound levels) by the Y-type barrier was compared with that by the vertical straight wall type noise barrier. The comparison proved that the Y-type noise barrier was rather greater in effect of noise attenuation than the vertical straight wall type one.
Also, a noise barrier having a limited height and showing an increased effect of noise attenuation is known from the disclosure in the Japanese Patent Application laid open as a Provisional Publication No. 85921/96. This conventional noise barrier is essentially composed of a main soundproof wall body extends upright from the ground, first and second additional screens provided on the top of the main wall body, the first additional screen being inclined toward a noise source while the second one is inclined away from the noise source. The first and second additional screens are provided thereon further additional screens, respectively, directed in different directions from those of the first and second additional screens, respectively. When this noise barrier structure is viewed from its lateral end, it looks like horns of a caribou. The noise barrier is called caribou-horn type. The caribou horn-type noise barrier structure can attenuate noise more effectively than the upright straight wall type and top-bent or -curved type noise barriers on the assumption that they have a same height.
The Y-type and caribou horn-type noise barriers can attenuate noise effectively with no increase of their height. Because of their complicated geometry such as their open-top design, however, these noise barriers are difficult to transport and install. Even if the additional screens are adapted for installation on the top of the upright straight main wall body at site, the members included in these noise barriers need a wide space for storage and transport, which will lead to a poor efficiency of installation and a greater total cost.